The objective of the studies outlined in this proposal is to investigate four fundamental aspects of the physiology and pathophysiology of autonomic regulation of glucagon secretion during hypoglycemia in nondiabetic animals and in animal models of diabetes. Hypoglycemia is a common and serious complication of insulin-treated diabetes mellitus in humans which limits the ability to attain improved glycemic control. The Diabetes Control and Complications Trial found a dramatic decrease of diabetic retinopathy and nephropathy with intensive therapy, but at a cost of a three-fold increase of severe hypoglycemia. Increased secretion of glucagon is a primary factor for recovery from insulin-induced hypoglycemia in nondiabetic humans. Activation of the autonomic nervous system has been demonstrated to make an important contribution to hypoglycemia-induced glucagon secretion in several species including dogs and rats, however, the role of the autonomic nervous system in humans is controversial and experiments of this type in nonhuman primates as models of human physiology have not been previously conducted. In diabetic humans, the glucagon and certain autonomic responses to hypoglycemia are often impaired. The etiology and time of onset of this impairment is poorly understood. Potential factors that may be involved include, but are not limited to, hypoglycemia-associated autonomic failure and autonomic neuropathy. Autonomic and glucagon responses to hypoglycemia are also known to be impaired in some animal models of diabetes, including diabetic rats, although few mechanistic studies have been conducted to examine the underlying etiology, nor has it been determined if pharmacological interventions can to prevent or decrease the counterregulatory defects. To address these deficits in the understanding of t he regulation of hypoglycemia-induced glucagon secretion: 1) Experiments will be conducted to examine the autonomic contribution to hypoglycemia-induced glucagon secretion in a nonhuman primate (rhesus monkeys) in the absence of diabetes. 2) To define the timing of the onset of impaired autonomic activation and glucagon secretion during hypoglycemia in rhesus monkeys with chemically-induced (streptozotocin) diabetes and the effects of different levels of metabolic control on the deficits. 3) A series of mechanistic studies will conducted in streptozotocin diabetic rats to investigate whether defects in of autonomic activation or reduced A-cell secretory responses to autonomic stimulation could contribute to impaired glucagon secretion and to determine the effects of different treatment regimens, designed to maximize chronic hyperglycemia or induce antecedent hypoglycemia, on autonomic responses and glucagon secretion. 4) Autonomic and glucagon responses to hypoglycemia will be examined in streptozotocin diabetic rats treated with pharmacologic agents that have been shown to ameliorate neural dysfunction in diabetic rats. Collectively, these experiments will lead to greater understanding of the pathophysiology and treatment of impaired hypoglycemic counterregulation in diabetes and the greater use of animal models for this area of investigation.